data binding with as3

I have a bit of a problem (since I'm not used to binding with AS3), the thing is that I want to do this ki开发者_运维知识库nd of data binding:

<mx:Application xmlns:mx="http://www.adobe.com/2006/mxml"
    layout="vertical"
    verticalAlign="middle"
    backgroundColor="white">

<mx:XML id="dp" source="countries_states_cities.xml" format="e4x" />

<mx:Form>
    <mx:FormItem label="Country:">
        <mx:ComboBox id="countryCB"
                dataProvider="{dp.country}"
                labelField="@name" />
    </mx:FormItem>
    <mx:FormItem label="State:">
        <mx:ComboBox id="stateCB"
                dataProvider="{countryCB.selectedItem.state}"
                labelField="@name" />
    </mx:FormItem>
    <mx:FormItem label="City:">
        <mx:ComboBox id="cityCB"
                dataProvider="{stateCB.selectedItem.city}"
                labelField="@name" />
    </mx:FormItem>
</mx:Form>

but the ComboBoxes are being created dynamically with AS3, everything is working except I cant bind the second ComboBox dataProvider to be the XMLList in the first ComboBox's selectedItem.

I guess you need runtime binding. It is done with BindingUtils class, see this for example. As for your case, it must be something like

BindingUtils.bindProperty(secondCombobox, "dataProvider",
    firstCombobox, "selectedItem");

Are you creating it dynamically based on data? If so, you could always use the Repeater component to repeat what you're trying to accomplish.

<mx:Repeater dataProvider="{someData}">
    <mx:FormItem label="{data.label}">
        <mx:ComboBox dataProvider="{data.stateCB.selectedItem.city}"
                labelField="@name" />
    </mx:FormItem>
</mx:Repeater>

Thanks, that is exactly what I was looking for, the code line that works for me is something like:

BindingUtils.bindProperty(comboBox, 'dataProvider', parentCB, {name: 'selectedItem', getter: function (host:ComboBox):XMLList {
                                var dp:XMLList;
                                if (host.selectedIndex >= 0) dp = host.selectedItem.children();
                                else dp = new XMLList();
                                return dp; 
                            }}, true);

However, I haven't been able to access the "site" inside the function, only the "host". Basically what I'm trying to do is make the second ComboBox start with selectedIndex = -1